1. Field of the Invention
The present invention pertains to a zoom lens system, and more particularly to a zoom lens system capable of covering all ranges including the wide-angle range and appropriate for a still camera, video camera, etc.
2. Description of the Prior Art
Conventionally, in a zoom lens system used in a still camera or video camera, it has been desired to increase the aperture ratio and magnification while reducing the number of lens elements in the entire system in order to make the camera less costly and more compact. In response to this need, a technology has been proposed that reduces the number of lens elements in the entire zoom lens system by employing a gradient index lens (GRIN lens) while performing aberration correction.
For example, Japanese Laid-Open Patent Application 2-79013 discloses an example of a zoom lens system comprising, from the object side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, wherein a GRIN lens is used in the second lens unit and both the first and second lens units comprise two lens elements.
Japanese Laid-Open Patent Application 2-56515 discloses an example of a zoom lens system comprising, from the object side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, wherein a GRIN lens is used in both the first and second lens units, each of which comprises a single lens element.
Japanese Laid-Open Patent Application 2-124509 also discloses an example of a zoom lens system comprising, from the object side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, wherein a GRIN lens is used in both the first and second lens units, each of which comprises a single lens element.
However, because the GRIN lens in the zoom lens system disclosed in Japanese Laid-Open Patent Application 2-79013 has spherical surfaces, there is insufficient freedom to perform aberration correction and each lens unit must comprise more than one lens element, and therefore the problem arises that the number of lens elements cannot be reduced any further.
With regard to Japanese Laid-Open Patent Applications 2-56515 and 2-124509, because the GRIN lens used in the zoom lens systems disclosed in said applications has spherical or flat surfaces, the problems arise that the total length of the entire lens system is large and that the lens system cannot be used for wide-angle purposes.